In defense of Leader
Content creators everywhere are complaining that Leader is too strong, that he needs to change, that he is simply "do whatever your turn is, but with 4 more power." This is simply not true, and my purpose today is to make the argument against this.
Before I begin, I want to acknowledge that there are multiple valid reasons to nerf a card. One of these reasons it the card is not fun to play against. At the end of the day, we are all playing this game to have fun, so if a card is that unfun...well, that is a valid reason to nerf it. The card I want changed the most is Galactus, and it is not because Galactus is too strong. It is because it is unfun for me to have to build every deck to play against him. If this is the reason that people think Leader needs a nerf, so be it...but I honestly think that if people do believe this, it is because they are drinking the Koolaid that plenty of creators are handing out and not really thinking for themselves.
Moving a step further, there is also the pairing with Leech. I will agree that when these 2 cards are paired together, some of the counterplay that I am about to delve into gets removed...because well, Leech removes abilities from all your cards. As such, I think Leech is quite unfun to play against - he removes that which is fun in card games - piecing together your cards in a way to find victory. On the other hand, I TRULY believe that Leader is fun to play against. And just for the record, I have been playing since closed beta began in May and today have an Ultra (red border) Leader with 3 unspent boosters - I am saying this to provide evidence that I am not sitting here and trying to defend the card "because you just like wining with it!!#$!@#!". I like winning against it!
Initiative
We begin with a look into initiative. This will be the first type of problem Leader has. On some other sites, there is content talking about "fighting for initiative on turn-5"...I understand why people are saying this, and if you go look at decks running Leader, you see cards like Lizard and Maximus being used alongside him...but it's a double edged sword for the Leader player. Initiative on turn-5 is for the Aero play, which can be strong. That said, it seems to me that more often than not, it is just slammed with little to no thought and the player loses (to me anyway) because of this initiative slam into Leader.
There are 2 different results for Leader, simply based on initiative. Pretend that you played Leader, and I played a Wolfsbane into a lane where I have 2 other units and you have 1 other unit. If you have initiative, your Wolfsbane's On-Reveal will trigger and she will gain 2 power, resulting in 3 power. If I have initiative, my On-Reveal will trigger, buffing Wolfsbane to 5 power. THEN, you will get a 5 power Wolfsbane who will have her On-Reveal trigger again and she will move up to 7 power. If you have initiative, you are simply 4 power lower. We could change the number of units on both sides to see this swing even further.
Similarly, if I played an Iron Man last turn and I have initiative, my Mystique will copy Iron Man, then you will get a Mystique-Iron-Man. If you have initiative, you will get a Mystique with who will check your last play before Leader for an Ongoing to copy...and very likely not find one. Meanwhile, I get my 2nd Iron Man still.
The next card you play (or when you play a card)...
These sorts of effects simply will not trigger for the Leader player. Angela and Bishop are the primary sources for this. While both Angela and Bishop are generally plays for earlier turns, Sera and/or Mr. Negative decks can play either card on turn-6. Regardless of initiative, these cards will not grow for the Leader player, but will for us. Titania will function as would be expected based on initiative. If she is the only card played in the lane by us, if the Leader player had initiative, theirs will reveal first, then when we play our Titania, the first (from Leader) will swap to our side. On the other hand, if we have initiative, both players will keep their Titania (unless Leader was played into the same lane, and he will trigger the swap).
Some other cards will simply not have their "next card you play" effect function if the Leader player had initiative. Examples being Shuri and Forge,
Cards that the Leader player just hates to see...
These are cards that just on their own break parity or actually harm the Leader player. Self-destruction cards are the most obvious (Destroyer, Carnage, Deathlok, etc). Theoretically, we wanted to have setup in prior turns with cards like Nova, Bucky, or Hood, but regardless, these break parity. Killmonger can as well. If we sequence Killmonger into Demon (from Hood), if our opponent has initiative, they will get the 2 cards (and both will live), then when our Killmonger flips, our opponent's Demon will die. After this, our Demon pops up.
Taskmaster is another card that automatically breaks parity, regardless of initiative in fact. It will always copy the last card played by the Leader player before Leader himself. While this can be a problem if opponent's turn-5 was a Shuri'd Aero or something - there are times this is simply not the case.
Mystique was already mentioned, and requires a lack of initiative to work well, but Absorbing Man and Rogue work in a similar manner. Furthermore, playing an On-Reveal into a Wong lane, will of course break parity.
Buffs are not created equal
I feel like this is obvious. Ka-zar, Silver Surfer, Patriot, Cerebro, even Blue Marvel. These should never be equal for both players. I mention Blue Marvel because the deck running Blue Marvel should already have a wider board than the Leader player, thus gain more from the +1 per unit. Hazmat is another, especially if Luke Cage was played earlier. The Leader copying Hazmat actively hurts the Leader player...it's even better when they copy that Debrii also. Oh, and Mysterio will be 3x 0-power units for the Leader player...
Realize that one part of these (either the buff or the recipient) needs to have been played prior to turn-6 to break parity.
Bringing it all together
Initiative is a huge deal. The reality is, that Leader players are looking to be ahead on turn-6 to play into Leader's upside potential. Thus, we are looking to do the opposite, and gain value from the lack of initiative. I've mentioned various ways that parity can be broken. Now, I am simply going to list a ton of cards that can break parity in one way or another. This should be considered starting well before turn-6, but in the current meta, you should have a plan to beat Leader, rather than simply complaining when you lose to it.
There are decks (Surfer Girls) that really just get hard countered by Leader..and in all honesty, this is GOOD. As long as I can remember, Leader was the inclusion in Surfer Girls to break the mirror. For all the complains of Leader right now, he is helping keep Surfer Girls in check. For what it's worth, turn-5 Wave is a counter to other decks, including plenty that I like to beat Leader with...
Onto that list of cards...
Absorbing Man, Angela, Ant-Man, Arnim Zola, Attuma, Beast, Bishop, Blue Marvel, Brood, Bucky Barnes, Captain America, Carnage, Cerebro, Cosmo, Darkhawk, Deathlok, Debrii, Destroyer, Devil Dinosaur, Doctor Strange, Dracula, Falcon, Forge, Ghost Rider, Hazmat, Heimdall, Hela, Ka-zar, Killmonger, Lizard, Lockjaw, Mr. Sinister, Morph, Mysterio, Mystique, Namor, Nova, Orka, Omega Red, Patriot, Polaris, Punisher, Red Skull, Rogue, Ronan, Shang-Chi, Shuri, Silver Surfer, Spectrum, Spider-Woman, Squirrel Girl, Strong Guy, Sunspot, Super Skrull, Taskmaster, Thanos, Collector, Hood, Thor, Titania, Typhoid Mary, Valkyrie, Viper, Warpath, Wasp, White Tiger, Wolfsbane, Wolverine, Wong, Yellow Jacket.
There's more, but this was the "quick list" I could throw together. They all have the ability to break parity...how easy it it to break said parity? Well, easier for some than other...but I find the challenge quite fun. While it takes 0 though to play Leader, there is a huge amount of skill expression in beating Leader. But yea, if you just jam a deck with 0 of the above cards and lose to Leader - that's on you, honestly.
If we pretend that Leader was deleted, I promise you that people would be back complaining about other cards and decks within 24 hours (my guess is the complaints would primarily about Surfer Girls w/ Aero). Leader helps keep other things in check and is generally telegraphed and not hard to identify. Add this with how beatable it is when we play any selection of cards that can beat the parity and he is honestly good for the health of the game.
As mentioned at the start, perhaps Leech changes this, and the combo is too oppressive. I'd say Leech is a better nerf target - not because of power, but because it fundamentally removes player agency and fun. Past that, I honestly believe that Galactus and Professor X are the next 2 cards that just make the opponent unable to interact and play the game, not Leader.